1. Field of the Invention
This invention pertains to control mechanism for effecting changes in gear ratios of a gear-change transmission. More particularly, it relates to a bicycle control mechanism which is mountable as a unit on any convenient portion of the bicycle, and which positively indexes displacement of a member for effectuating changes in gear ratios in response to unidirectional movement of an operator control lever.
2. Description of the Prior Art
Several known mechanisms and methods exist for controlling the operation of bicycle gear-change transmissions. The more conventional units are operable on the well known chain-derailleur principle. Other units function on the principle of sliding gears, or moving elements which may comprise sprockets, or the like. All of the above mechanisms and methods of operation, however, share a common characteristic in that the change of gear ratios is effected by motion of the elements which is controlled by the displacement of a tensioned flexible cable or the like.
Present day bicycle control mechanisms which may accomplish such aforenoted types of changes in gear ratios are broadly classifiable into several general categories. One such category utilizes a friction-loaded lever-and-drum arrangement which is operable to effect control over the displacement of a tensioned cable. In this particular kind of control mechanism, no fixed gear positions are provided, and as a consequence the rider must shift by feel while at the same time making minute corrections until signs of improper alignment are no longer evident. The rider in shifting must also overcome the ordinarily high amount of friction imposed on the shift lever. Shifting gears with known types of mechanisms is therefore not entirely satisfactory, and involves in many instances an undue amount of time and effort.
In seeking to obviate such shortcomings, longer levers and reduced cable drum diameter have been used. Such mechanisms, however, have resulted in rather large total lever motion ranges which necessarily restrict the choice of mounting positions available on the bicycle. As a result, in certain of the friction-loaded lever-and-drum arrangement type of control mechanisms the rider is required to remove his hand from the handlebars in order to shift the gear ratio. Obviously, such an arrangement for shifting is relatively unsafe, not to mention inconvenient and troublesome in operation.
Another common type of present day bicycle control mechanism operates by virtue of a spring-detented lever-and-drum arrangement which also is effective to control the displacement of a cable to effect a desired shift in the gear ratio. Although this type differs from the aforementioned type in that fixed gear positions are provided, it nevertheless suffers from certain other drawbacks. That is, a detent force of a relatively large magnitude is ordinarily required to resist cable tensions that may occur in normal operation. However, such a detent force creates a condition which requires a relatively high degree of rider effort to accomplish a shift in gear ratio. Moreover, care must be exercised by the rider so as to avoid overshooting the intended gear during shifting. As a result, a less than completely satisfactory control mechanism is provided.
Still another type of conventional bicycle control mechainism incorporates a ratchet mechanism, wherein the operating or gear shift lever follows the position of the cable drum as the former moves from position to position. This latter type of mechanism, however, is primarily used only in conjunction with the internal-hub gears of the planetary type, and, has not been successfully applied to the typical chain-derailleur system type of gear-change transmission. Furthermore, the range of positions attendant with the use of such an operating mechanism necessitates rather large total lever motion which correspondingly restricts quick rider access to the lever by reason of the uncertainty as to its location at any given time. In addition, such types of lever arrangements further restrict the available mounting positions on the bicycle, and quick rider access to the shift mechanism for the bicycle transmission is hindered and inconvenient.
An example of the last noted type of shift device for a bicycle gear-change transmission is disclosed in U.S. Pat. No. 2,785,586. Such type is effective to control the selector of variable speed gears for motor vehicles by a pulling action exerted on a Bowden cable. However, in such a device, by virtue of its construction, rather high impact loads on the pawls and ratchet teeth may be readily expected. Moreover, its manually operated control lever requires bidirectional movement to perform upshift and downshift operations. Hence, discomfort and loss of control may be encountered when an operator moves his hand around to the other side of such lever so as to bring about a particular shift. Further, if such a device is to be used with five or six-speed derailleurs, for example, tooth height would change cumulatively.
In addition to the aforenoted problems, it has been determined that known types of gear-change transmissions which utilize a tensioned flexible cable to control displacement of a biased transmission member to effect a change in gear ratios may suffer certain shortcomings in that they are subject to cable stretch. As cable stretch occurs, it becomes more difficult to accurately control the movement of such transmission member. Moreover, cable stretch is more pronounced whenever cable length is relatively large. Hence, if a control mechanism is mounted on the handlebars, accurate movement of the transmission is more difficult to obtain. Additionally, a Bowden type cable is subject to friction losses which may also tend to make accurate control of the cable more difficult.
A bicycle gear-change transmission which includes a chain derailleur type of mechanism also experiences a degree of reluctance which is created by the bicycle sprocket chain. That is, chain reluctance normally occurs when the spring-biased cage which mounts the sprocket chain is pivotally movable laterally with respect to the principle axis of the sprocket chain so that the sprocket chain moves to a gear sprocket having a larger diameter. During such travel, the chain sprocket must be forced so that the chain will be in proper alignment with the desired sprocket. The intended action, however, may be hindered because the sprocket chain has a tendency to remain on its present sprocket which is sufficient to impair the desired relatively smooth and quick transfer of such chain from one gear to the next. When the cage is moved far enough to cause the chain to shift, the chain will not be properly aligned with the new sprocket, having been moved too far. Such alignment may cause the chain and gear sprocket to function in a noisy and self-destructive manner. Mention should be made, however, that the above problem concerning chain reluctance does not exist when the sprocket chain is laterally moved to a sprocket having a smaller diameter.
Attempts to prevent such sprocket chain misalignment have resulted in the necessity of "overshooting" the desired chain position. Overshooting is achieved by having the sprocket chain laterally moved a certain distance past the position for the intended gear sprocket. In this manner, the chosen sprocket chain is enabled to ride up and then drop onto the gear teeth of the adjacent larger sprocket. However, after each shift, it is necessary to draw the derailleur mechanism back into a position of alignment. Moreover, initial overshooting must be relaxed as soon as the chain has commenced to shift. It will therefore be appreciated that some skill and constant attention on the part of the operator during the entire course of the shift must be exercised.